1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission, and a method of controlling a hydraulic pressure of the automatic transmission. More specifically, the hydraulic control system under the present invention controls, as a standby pressure, a pressure which is supplied to friction elements from a point in time at which an engagement instruction is started to a point in time at which an abutment (an input element abutting on an output element) is started, when at least one of the friction elements is switched from a disengagement state to an engagement state so as to carry out a power train.
2. Description of the Related Art
U.S. Pat. No. 5,941,795 {equivalent of Japanese Patent Unexamined Publication No. Heisei 9(1997)-273626} discloses a hydraulic control system for an automatic transmission. The hydraulic control system according to U.S. Pat. No. 5,941,795 increases a clutch pressure supplied (applied) to a predetermined clutch (friction element), to thereby switch the clutch from a disengagement state to an engagement state. With the switching, the hydraulic control system transmits revolution of an input element to an output element of the clutch. Herein, the hydraulic control system controls the clutch pressure in accordance with operation of the clutch, which clutch pressure is supplied from a point in time at which the engagement instruction is started to a point in time at which the engagement is terminated.
The hydraulic control system according to the related art, however, occasionally causes the following inconvenience when the clutch revolution speed is high:
As is seen in FIG. 8, there is provided a time chart showing schematically an operation of the hydraulic control system, according to the related art. The hydraulic control system switches the clutch from the disengagement state to the engagement state by increasing the clutch pressure P-C supplied (applied) to the clutch. Thereby, a routine of the hydraulic control system carries out an instruction as is indicated with a broken line P-I(0), in accordance with the following three steps:
1. The routine controls the clutch pressure P-C at a pre-charge pressure Pr in a predetermined period from an engagement start point t0 (at which the engagement instruction is started), to thereby shorten the time for charging the clutch with operation fluid.
2. As a standby pressure PS(0), the routine controls the clutch pressure P-C supplied in a period from a termination point t1 (at which the pre-charge pressure control is terminated) to a time point t2 (at which an abutment between the input element and the output element is started). Hereinafter, the time point t2 is referred to as xe2x80x9ctorque phase start point t2.xe2x80x9d The routine allows a clutch piston to make a stroke to such an extent that the input element and the output element of the clutch are on the verge of the abutment on each other.
3. The routine makes such an instruction as to increase rapidly (skyrocket) the clutch pressure P-C from the toque phase start point t2. Thereby, the input element abuts on the output element, to thereby engage the clutch.
In the case of the hydraulic control described above, the clutch pressure P-C(Low) at low clutch speed is supplied in a manner indicated with a one-dot chain line in FIG. 8. Moreover, as is indicated by a first region xcex1, the clutch pressure P-C(Low) at low clutch speed causes a surge pressure at the torque phase start point t2.
Contrary to this, in spite of the same instruction pressure P-I(0), the clutch pressure P-C(High) at high clutch speed is supplied in a manner indicated with a two-dot chain line in FIG. 8. More specifically, as compared with the clutch pressure P-C(Low), the clutch pressure P-C(High) is retarded with respect to the instruction pressure P-I(0). In addition, as described above paragraph, when the clutch speed is low, the surge pressure is caused at the torque phase start point t2 (see the first region xcex1). Contrary to this, when the clutch speed is high, the surge pressure is retarded (see a second region xcex2). In addition, the surge pressure at high clutch speed is higher than the surge pressure at low clutch speed. This paraphrases that a small shock is caused when the clutch speed is high.
When a friction element of an automatic transmission is at high revolution speed, speed of supplying operation fluid to the friction element is likely to be retarded.
It is therefore an object of the present invention to provide a hydraulic control system for an automatic transmission preventing such retardation.
According to a first aspect of the present invention, there is provided a hydraulic control system for an automatic transmission. The automatic transmission includes a plurality of friction elements. At least one of the friction elements is supplied with a hydraulic pressure which is so increased as to switch the friction element from a disengagement state to an engagement state, thus transmitting a revolution of an input element of the friction element to an output element of the friction element. The hydraulic control system controls the hydraulic pressure as a standby pressure in a period from an engagement start point at which an engagement instruction for engaging the friction element is started to an abutment start point at which an abutment between the input element and the output element is started. The hydraulic control system comprises: a speed sensor for sensing a speed of the friction element; and a standby pressure increase section for increasing the standby pressure in accordance with the speed of the friction element sensed with the speed sensor.
According to a second aspect of the present invention, there is provided a method of controlling a hydraulic pressure with a hydraulic control system for an automatic transmission. The automatic transmission includes a plurality of friction elements. At least one of the friction elements is supplied with the hydraulic pressure which is so increased as to switch the friction element from a disengagement state to an engagement state, thus transmitting a revolution of an input element of the friction element to an output element of the friction element. The hydraulic control system controls the hydraulic pressure as a standby pressure in a period from an engagement start point at which an engagement instruction for engaging the friction element is started to an abutment start point at which an abutment between the input element and the output element is started. The method of controlling the hydraulic pressure comprises the following steps of: sensing a speed of the friction element with a speed sensor; and increasing the standby pressure, with a standby pressure increase section, in accordance with the speed of the friction element sensed at the sensing step with the speed sensor.
According to a third aspect of the present invention, there is provided a hydraulic control system for an automatic transmission. The automatic transmission includes a plurality of friction elements. At least one of the friction elements is supplied with a hydraulic pressure which is so increased as to switch the friction element from a disengagement state to an engagement state, thus transmitting a revolution of an input element of the friction element to an output element of the friction element. The hydraulic control system controls the hydraulic pressure as a standby pressure in a period from an engagement start point at which an engagement instruction for engaging the friction element is started to an abutment start point at which an abutment between the input element and the output element is started. The hydraulic control system comprises: a sensing means for sensing a speed of the friction element; and an increasing means for increasing the standby pressure in accordance with the speed of the friction element sensed with the sensing means.
The other objects and features of the present invention will become understood from the following description with reference to the accompanying drawings.